The effects of blue-light on organisms ranging from microorganisms to man have been recognized for more than 100 years. The known effects of blue-light range from DNA repair to inducing the expression of certain genes. In humans, blue-light may be involved in setting the biological clocks or circadian rhythms. The long term goals of this research are to elucidate the biological roles fulfilled by the DNA photolyase/blue-light photoreceptor family of genes in the model organism, Chlamydomonas, a single-celled alga. Two genes encoding members of the DNA photo lyase/blue-light photoreceptor family have been cloned from Chlamydomonas. The first gene isolated, named CPH1, is hypothesized to be a blue-light photoreceptor. The second gene, named PHR2, encodes a protein with DNA photolyase activity and is hypothesized to be the photolyase targeted to the chloroplast. A mutation, phr1, in a third gene that is unlinked to the other two genes results in a deficiency of DNA photolyase activity in the nucleus and is hypothesized to encode a third member of this family of proteins. In this proposal we will test the hypothesis that CPH1 encodes a blue-light photoreceptor by genetically altering the expression or function of the protein and measuring the subsequent effects on genes whose expression is controlled by blue light. Using an antibody to CPH1, the amount and cellular localization of this protein will be determined under dark and light grown conditions. An attempt will be made to isolate proteins that physically interact with the CPH1 protein as they will be candidates for components of the signal transduction pathway initiated by the absorbance of the blue light. The hypothesis that the PHR2 protein is targeted to the chloroplast will be tested by in vitro uptake studies. The final immediate goal is to isolate the PHR1 gene using the tools of insertional mutagenesis, complementation of photolyase mutants with a cDNA library, or PCR based on sequence homology to other members of the family.